Total hip replacement, through the use of a hip prothesis, is one of the most successful recent medical innovations. However, it has become apparent that in the long term, an implanted hip prothesis commonly encounters problems. The most common form of implant failure, once the medical operation of insertion of the implant has successfully been completed, is eventual loosening of the implant within the femur. While revision surgery is possible, success rates are much lower than in primary cases and the incidence of complications (such as infection, dislocation and loosening) following the new surgical operation is higher. Failure often occurs earlier in revision cases.
The loosening of the hip prothesis is a multi-factorial process. Mechanical factors, related to stress distribution within the femur bone itself, and biological factors relating to resorption of the bone of the femur, both have a role.
Typically, prior to implanting a femoral prothesis, the head of the femur has been removed by cutting across the neck. The femoral prothesis is then inserted into the femur which has a profound effect upon the stress distribution within the proximal femur. In a normal proximal femur weight bearing is transferred directly to the thick cortex of the medial femoral neck, or calcar femorale. In contrast, after implantation of a conventional hip prothesis, most of the weight bearing by-passes this portion of the femoral neck and is transferred distally to the tip of the implant stem.
In experimental study in relation to conventional hip inserts, it has been found that a massive decrease in stress in the region of the thick cortex of the medial femoral neck occurs after implantation (Oh and Harris; Proximal Strain Distribution In The Loaded Femur: Journal of Bone And Joint Surgery. 60A - : 75 January, 1978). The study concluded that this so called stress shielding is likely responsible for resorption of the thick cortex of the medial femoral neck which is seen in up to 70% of hip replacements.
There have been attempts in the prior art to restore loading to the cortex of the medial femoral neck by adding a collar to the femoral prothesis. However, the prior art devices rely on some form of attachment between the femoral insert and the inner surface of the femur such as by using wedge shaped stems or cementing or bonding the stem of the femoral insert to the femur itself. This makes it difficult, if not impossible, to ensure that a good contact is made between the collar and the thick cortex of the medial femoral neck. In addition, it fails to transmit forces adequately to the calcar femorale. Further, a small amount of resorption, induced by for example, surgical trauma, stress shielding or vascular interruption could unseat the collar thereby making the collar useless.
Therefore, what is desirable, is to provide a femoral prothesis which will reliably transfer the bulk of weight bearing directly to the thick cortex of the medial femoral neck.